The Alpin 100MKIII is a rugged HF+ 6m linear amplifier that weighs in at 26kg. This meaty unit is completely self-contained with AC power supply and the rugged GU74B/4CX800 tube responsible for delivering the power.

Anybody considering the purchase of an amplifier are faced with a number of choices and directions to go in, and probably the most fundamental is whether to choose solid state or valve. There are still more valve amplifiers about today, even though almost every transceiver now uses a solid state PA device (or devices). And there is probably good reason for the retention of the valve for high power use.

A rugged self contained amplifier with high levels of protection.

The Alpin 100MKII is a rugged HF+ 6m linear amplifier that weighs in at 26kg. This meaty unit is completely self-contained with AC power supply and the rugged GU74B/4CX800 tube responsible for delivering the power.

Anybody considering the purchase of an amplifier are faced with a number of choices and directions to go in, and probably the most fundamental is whether to choose solid state or valve. There are still more valve amplifiers about today, even though almost every transceiver now uses a solid state PA device (or devices). And there is probably good reason for the retention of the valve for high power use.

A valve employs a high plate voltage to deliver the power and gain and this results in a relatively low current. It is much easier to obtain a clean output with high voltage and low current swing, and also enables the circuit to employ the well established pi-network variable output tuning which in turn makes it easy to deliver full power into modest VSWRs. On the other hand, a solid state PA operates on a very much lower voltage and to deliver the power and gain, it must draw high currents. This makes it more difficult to generate a clean signal and requires more extensive filtering. It also precludes the use of any form of variable output matching such as the pi-network just referred to. Other considerations are the fact that changing a valve is usually easy, but changing a solid state device often requires major surgery and is not a job that most owners would want to undertake for themselves.

If all this sounds as if the balance weighs heavily in favour of the tube, the answer is yes, for many people, but of course there are many solid state linears in use very successfully. But there is one issue with a tube design that has in recent times become an issue. Many operators have been brought up on a diet of solid state technology and so do not have an appreciation of the way in which a valve operates and the risk of valve damage that can occur in the hands of the inexperienced user. This mainly relates to the tuning and adjustment of the amplifier, where over driving and periods of off-resonance and high currents can result in damage to the tube. This is where a modern design like the Alpin-100 has added significant improvements and safety measures to the design of the traditional valve amplifier.

In the case of the Alpin-100MKII there is no traditional meter on the front panel, this being replaced by an LCD panel that provides all the information. When initially tuning, a 6dB attenuator is switched in until tuning is approaching the optimum and the LCD helps by indicating in which direction the tuning control needs to be rotated. Further protection is offered in the form of over drive and also a system whereby fault conditions can be displayed. Areas that are monitored are: Forward & Reflected power, Drive level, HF voltage, Current (Anode Screen and Grid) exhaust air temperature etc., The ultimate objective being to provide a design that is safe and easy to operate.

The amplifier will run to full power output of 1.3kW PEP when driven with around 65 Watts input. The amplifier is grid driven across a resistive load and so no input tuning is needed. For CW and RTTY the amplifier is rates at 1kW continuous and these levels offers plenty of headroom for working within the UK. RF switching is rapid, using the normal PTT to ground system and an internal vacuum relay provides the speed and power capability.